1. Field of the Invention
This invention relates to video signal processing circuits and, in particular, to such circuits which are used in the luminance signal amplifying channel of a television receiver.
2. Description of the Prior Art
In a typical television receiver, it is normal to employ separate brightness and contrast controls whereby these respective characteristics of the image displayed upon the receiver's cathode ray tube (CRT) may be adjusted. Typically, the contrast control is inserted in the luminance signal amplifying channel or video signal amplifier in a manner to adjust the amplitude of the video signal. In FIG. 3, a signal horizontal scan line of the video signal is shown, illustrating the video information signal as having a peak-to-peak amplitude A occurring during the horizontal trace period, and the sync tip, white level , and back porch portions of the signal. It is understood that varying the contrast control would set or readjust the amplitude A; for example, if it is desired to diminish the contrast of the displayed picture, the amplitude A is decreased. On the other hand, the separate brightness control adjusts the black level of the video signal to determine the point at which cutoff occurs of the electron beam of the CRT. Typically, the brightness control in the form of a variable potentiometer is coupled to set to the DC bias applied to the output, driver transistor and in particular the DC bias as applied to its base.
In a typical TV receiver, the output, driver transistor is AC/DC coupled with either the control grid or cathode of the CRT so that adjusting of the DC bias applied to the output transistor in turn adjusts the black level of the video signal, which approximates the back porch level of the video signal. Thus, where the amplitude A is decreased from a situation of high contrast to a situation of low contrast, the DC bias remains relatively constant so that a corresponding change in the white peak level and the black level, also will occur. As a result, when the contrast is readjusted as desired, the black level is, in a sense, lowered so that the displayed black portions of the image now appear as gray. To correct this condition in a normal TV receiver, the brightness control is adjusted to reset the black level to a relatively constant level so that the black portions will appear as black. Thus, in order to maintain the quality of the displayed image in terms of maintaining the blackness of the dark portions of the image, it is necessary to adjust two distinct controls.
In U.S. Pat. No. 3,804,981 of Avins, there is disclosed a luminance signal amplifier including separate contrast and brightness controls. In particular, it is disclosed that the contrast control in the form of a potentiometer sets the gain of an amplifier in the form of a driver transistor, which variably amplifies the video input signal as fed through its base, and the separate brightness control sets the DC bias level of the output, i.e. the collector, of the driver transistor. Further, the contrast and brightness controls, each in the form of potentiometers, are intercoupled by a coupling resistive element so that the black level or brightness can be held constant or made to vary in a manner as determined by the setting of the contrast control. The above-noted patent indicates that the direct voltage level at the output of the driver transistor does change if the contrast control is varied and seeks to compensate for such change by the insertion of the coupling resistor. However, the readjustment of the black level is made dependent upon the setting of the contrast control so that, in effect, the black level is not set with respect to a relatively constant level, but rather to the setting of the contrast control itself. Thus, it is contemplated that for certain settings of the contrast control, the DC level of the output of the driver transistor would result in the display of an image, wherein the black tones are gray values, e.g. for relatively low settings of contrast.